Since the early 1960s there has been a strong connection between surfing and skateboarding that has influenced the history of skateboard design as well as the larger culture of board sports. Terms like “sidewalk surfing” and “surfing-like ride and feel” speak to this connection and have been used to describe ways skateboards attempt to mimic the sensations and ride dynamics of surfing.
Surfing, and other board sports share common ride dynamics of deep deck lean, stability at speed and the ability to “carve” turns.
“Carving” is the ability to make turns and control speed and is associated with deep deck lean and a feeling of “sinking into” the turn such that the deeper the deck or board is leaned the stronger the carving sensations. Carving turns typically involves higher speed and higher turn forces that must be matched by rider input, commitment, and advanced skill. With carving there is also a weightless, floating sensation experienced in the transition between linked turns.
The lean-steering mechanism of skateboards, skates and the like is commonly referred to as the “truck-assembly”, or simply a “truck”. A skateboard truck typically comprises two rigid bodies generally referred to as a baseplate and a hanger where the baseplate is mounted to a deck and the hanger supports two laterally spaced wheels that roll on the ground. The rigid bodies of baseplate and hanger are kinematically linked so as to allow rotation relative to each other about a common axis defined by the geometry of the baseplate called here the “hanger pivot axis”.
A skateboard typically comprises a deck upon which the rider stands and a pair of trucks symmetrically mounted to each end of the deck. So constrained by the plane of the ground, a rider standing on the deck leans the deck right to steer right and left to steer left.
Existing skateboard trucks known as fixed kingpin trucks consist of mechanisms with two rigid bodies. The present invention introduces a new class of skateboard truck with three rigid bodies, an additional degree of freedom that is not present in existing skateboard trucks, and three primary motions which provide deep deck lean, improved steering control, improved stability at speed, and improved suspension. By delivering this combination of functional attributes the present invention is thus of great use to skateboard riders in search of a more powerful surfing-like ride feel.
Fixed Kingpin Trucks—Kinematic Description (Description of Space Mechanism)
As shown in FIGS. 2-5, fixed kingpin trucks of prior art are a class of trucks that utilize two rigid bodies: (1) a baseplate/fixed kingpin 901 and (2) a hanger 902.
Kinematic diagram FIG. 4 shows the pair of rigid bodies: baseplate/fixed kingpin assembly 901 and hanger 902 connected by semi-spherical joints 911 and 912.
A “hanger pivot plane” 910 is a central longitudinal plane of baseplate/fixed kingpin assembly 901 perpendicular to top mounting surface 904 of baseplate 905 as shown in FIG. 5 and coincident with the axis 925 of fixed kingpin 915. With fixed kingpin trucks the hanger pivot plane remains perpendicular to the baseplate and coincident with the centroid point of the first and second semi-spherical joints 911 and 912.
Hanger pivot axis 914 is defined by the centroid points of the first and second semi-spherical joints 911 and 912 and coincidence with hanger pivot plane 910. With fixed kingpin trucks hanger pivot axis 914 provides a single degree of freedom about which the pair of rigid bodies baseplate/fixed kingpin assembly 901 and hanger 902 rotate relative to each other.
A “hanger pivot axis angle” 919 is defined by the inclined angle of the hanger pivot axis 914 relative to the top surface 904 of baseplate 905 that supports the skateboard deck.
A “virtual pivot point” 916 is located at the intersection of the hanger pivot axis 914 and line 926 vertically projected from the center of hanger axle axis 913. The assembly of a skateboard with two fixed kingpin trucks creates a single deck roll axis called here the “virtual pivot point roll axis” 917 that is defined by the virtual pivot points 916 of the front and rear trucks.
As described fixed kingpin trucks can be understood as a space mechanism with two rigid bodies and a single degree of freedom.
Fixed Kingpin Trucks—General Description
FIG. 5 shows the typical features and assembly of a fixed kingpin truck. Baseplate 905 has a recess 918 with a plastic pivot cup insert that receives the end of the pivot arm 920 of hanger 902 to form a first semi-spherical joint 911. Baseplate 905 contains kingpin 915 that extends downward at an inclined angle. The kingpin 915 is typically fixed to baseplate 905 by press fit, threaded, or bolted connections, and therefore functions as a single rigid body called here baseplate/fixed kingpin assembly 901.
Hanger 902 has a pivot arm 920 and a centrally positioned, ring-shaped yoke 921 that receives the fixed kingpin 915. When assembled the ring-shaped yoke of the hanger is sandwiched between elastomeric bushings 922 and 923 to form a second elastomerically constrained spherical joint 912. The elastomeric bushings are integral to truck assembly and provide a return-to-center force.
Hanger axle members 903 support a pair of laterally spaced wheels. That assembly is typically completed by tightening the kingpin nut 924 to preload the elastomeric bushings 922 and 923 and constrain the yoke surfaces of the hanger with the fixed kingpin 915. Tightening the kingpin nut also constrains the first semi-spherical joint 911 of the baseplate pivot cup 918 and hanger pivot arm 920 from coming apart.
Fixed Kingpin Trucks—Kinetic Description (Description of Forces that Cause Motion).
In use a rider stands on the deck of an assembled skateboard and the wheels are constrained by the plane of the ground. On a skateboard with fixed kingpin trucks rider input to lean the deck directly causes the rotation of the deck and baseplates and hanger pivot plane 910 to rotate about the virtual pivot point roll axis 917 and the hangers to rotate about the hanger pivot axis of each truck resulting in the classic lean-steering response of the wheels on the ground. Springs or elastomeric components provide a return to center force.
Fixed Kingpin Trucks—Ride Dynamics.
Fixed kingpin trucks typically have a limited range of adjustment which is not ideal because the firmness of the elastomeric bushing and preload adjustment must match both rider weight and specific style of riding. As well, fixed kingpin trucks with bushings that are too soft for rider weight, are worn, or are too loosely adjusted become unstable at higher speeds. Consequently, riders must carefully choose between bushing durometer and preload adjustments that favor deeper deck lean and turning at slower speed or limited deck lean and greater stability at higher speed.
Design and geometry of fixed kingpin trucks have become specialized and optimized for specific speed ranges requiring riders to choose between (1) fixed kingpin trucks optimized for deeper deck lean or (2) fixed kingpin trucks optimized for stability at higher speed that consequently have a limited range of deck lean and do not turn well at slower speed.
With this specialization, skateboards with fixed kingpin trucks are not able to deliver the combined functionality of deep deck lean, turning, and stability across all speed ranges.